1. Field of the Invention
The present invention relates to an exhaust gas recirculation (EGR) system having an EGR cooler in an EGR passage, through which part of exhaust gas is recirculated to an intake passage. The EGR cooler cools the recirculated exhaust gas.
2. Description of Related Art
An exhaust gas recirculation system (EGR system) is commonly used for reducing emission of a diesel engine. The EGR system recirculates part of exhaust gas to an intake passage. EGR gas, which is the exhaust gas recirculated to the intake passage, includes much inert gas such as vapor or carbon dioxide, which is generated in combustion. An advantage of the EGR system is that generation of nitrogen oxide is inhibited because combustion temperature is decreased. If an EGR cooler is disposed in an EGR passage for cooling the EGR gas, charging efficiency of the EGR gas is improved, and the emission-reducing effect is further improved.
However, if the EGR system is used for a long time, soot or unburned hydrocarbon included in the EGR gas may adhere and deposit onto a heat exchanging part of the EGR cooler. The deposition of the soot or the hydrocarbon decreases heat-exchanging efficiency and cooling performance. As a result, the emission-reducing effect is decreased.
It is therefore an object of the present invention to eliminate soot or unburned hydrocarbon adhering to a heat-exchanging part of an exhaust gas recirculation (EGR) cooler of an EGR system for an internal combustion engine. Thus, degradation of cooling performance is prevented, and a high emission-reducing effect is maintained for a long time.
According to an aspect of the present invention, an exhaust gas recirculation (EGR) system of an internal combustion engine has an EGR cooler disposed in an EGR passage. The EGR passage connects an exhaust passage with an intake passage in order to recirculate part of exhaust gas. The EGR cooler cools EGR gas passing through the EGR passage. The EGR system has cooling performance detecting means and cooling performance regeneration controlling means. The cooling performance detecting means detects cooling performance of the EGR cooler. The cooling performance regeneration controlling means performs a regenerating operation for regenerating the cooling performance when degradation of the cooling performance is detected.
Thus, the EGR cooler can maintain excellent heat-exchanging performance even if the EGR cooler is used for a long time. As a result, an emission-reducing effect is maintained for a long time.
The EGR system has intake pressure measuring means for measuring an intake pressure. The cooling performance detecting means determines that the cooling performance is degraded when the intake pressure measured by the intake pressure measuring means is lower than a normal intake pressure by at least a predetermined value. The normal intake pressure is estimated from an operating state of the engine. If the unburned ingredients such as the soot deposit on the EGR cooler, a passage area and a flow rate of the EGR gas are decreased. Accordingly, the intake pressure decreases. Therefore, the degradation of the cooling performance can be determined based on the decrease in the intake pressure.